1. Field of the Invention
This invention relates primarily to magnetic resonance imaging systems. In a primary application this invention relates to the use of radio frequency fields, including gradients to rotate the magnetic moments while receiving signals in the rotating frame.
2. Description of the Prior Art
Magnetic Resonance Imaging has become one of the wider-used modalities in the field of medical imaging. A descriptive series of papers on NMR imaging appeared in the June 1980 series of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are covered in the lead article, "Introduction to the Principles of NMR" by W. V. House, pp. 1220-1226.
In general, in a MRI imaging system, the object being studied is within a highly-uniform intense static magnetic field. The object is then excited by a high-power radio frequency burst which causes the magnetic moments in the object, which were lined up with the static field, to precess normal to the static field. Using spatially orthogonal gradients, these magnetic moments become spatially varying. A receiver coil picks up the signals from the precessing moments. This signal is processed to create images of the magnetic moment density in the object.
These systems have a large number of theoretical and practical problems. The resulting costs are quite high since a highly uniform magnet is required over a relatively large volume. Also, a high-power radio frequency transmitter is required which must be uniformly distributed over the object and must avoid any excessive heating effects. Anything in the object which modifies the magnetic field, such as materials within the body which have changes in magnetic susceptibility, can seriously distort the image. Slightly differing magnetic resonances from different materials, primarily water and fat, become translated from each other, distorting the image. Also, many solid materials cannot be imaged since, in the presence of a strong polarizing field, the take on very short decay times. MRI machines cause loud sounds when the gradient coils are excited in the presence of a static magnetic field. Another practical difficulty with existing instruments is that high-power wide bandwidth amplifiers are required to run the gradient coils.
Efforts at increasing the SNR (signal-to-noise ratio) of NMR images usually involve increasing the magnetic field strength. However, in existing systems this ability is quite limited. If the field is increased, the r.f. excitation frequency must also be increased, greatly aggravating the r.f. heating problem. Also, the penetration of the r.f. signal on both the transmit and receive modes becomes a serious problem.
Perhaps the most important of the economics is that MRI, despite its almost ideal lack of toxicity and radiation, is not used for mass screening for any disease because of the prohibitive costs involved.
One attempt at a simpler system is given in a paper by J. Stepisnik, M. Kos, and V. Erzen in Proc. of XXII Congress Ampere, Roma, 512, 1986. Here the magnetic field is pulsed and then the magnetic moments are allowed to line up in the earth's field. Following this, an r.f. excitation is used to rotate the moments, with a set of gradients in the sane direction as the earth's field used to create an image. This system has marginal performance, given the very weak gradients, and limited economic advantage, since r.f. excitation is required. A non-imaging system, without any r.f. excitation, was used to measure the earth's field using NMR. This system is described in Phys. Rev., A94, 941, (1954). Here a water sample is subjected to a pulsed field normal to the earth's field. The pulse is shaped so that the magnetic moments remain in the direction of the pulsed field when it turns off. Following the pulse, the precession frequency due to the earth's field is measured to determine the earth's field. This system used no gradients and did not provide imaging.
U.S. Pat. No. 5,057,776 by the applicant, disclosed a system using a pulsed field to polarize the moments followed by a bias and gradient fields in quadrature to cause precession. No rf excitation was required. A recent application by the applicant entitled PULSED FIELD MRI SYSTEM WITH SPATIAL SELECTION introduced slice-selection mechanisms for this system. A copending application by the applicant on this same system entitled MRI IMAGING SYSTEM USING A PERIODIC BIAS FIELD introduced a cyclical bias field for further immunity to inhomogeniety.
A few papers have been written on the general subject of using the rotating frame of the NMR system to provide imaging. An early paper by D. I. Hoult (J. Magn. Reson., vol 33, p. 183, 1979) used the rotating frame to provide spatially selective excitation, with the readout done in conventional fashion. S. J. Cox and P. Styles (J. Magn. Reson., vol 40, p. 209, 1980) used rotating rf gradients for chemical shift imaging. The most direct use of rotating frame imaging came in a paper by F. De Luca (Lettere Al Nuovo Cimento, vol 39, p. 390, 1984) in which he suggested the use of three dimensional rf gradients plus an rf bias field and imaging in the presence of the rf excitation. This system has never been used or tried for a number of reasons. At normal magnetic fields, the rf power deposition would be excessive and far beyond present standards. If the static magnetic field is reduced to a lower level to avoid rf heating, the SNR would be unacceptably low. Also, the system is susceptible to both rf and static field inhomogeniety making it difficult to construct. Finally, the system described showed no method of selective excitation to limit the imaging volume.
In the early pre-imaging era of MRI the concept of "rotary echoes" was introduced which is related to the general problem of inhomogeniety, a subject of this application. This is described in a paper by I. Solomon in Phys. Rev. Letters, vol. 2, p. 301, 1959. The material is repeated in Ch. III p. 70 of A. Abragam, Principles of Nuclear Magnetism, Oxford Press, reprinted 1985. Here the rf field B.sub.1 is reversed at specific times to create a sequence of echoes when the moments return to the z axis. As indicated, this is a non-imaging experiment without gradients.